Retinol (compound 1) and its metabolites are involved in regulating many biological processes including vision, cell differentiation, and growth. Besides being essential to normal cell function, the retinol metabolite all-trans retinoic acid (RA, 2) also shows antiproliferative action in cancer.1 At pharmacologically effective doses, however, RA causes severe toxicity. Therefore, development of retinoid analogs possessing a higher therapeutic index is needed. One of the most investigated synthetic retinoids is N-(4-hydroxyphenyl) retinamide (4-HPR; compound 3), which has been shown to be effective in numerous types of animal tumor models and has been evaluated in a phase III clinical trial.2 A possible benefit was reported for the prevention of second breast malignancy in premenopausal women with surgically removed stage I breast cancer or ductal carcinoma in situ. Although 4-HPR is generally well tolerated, it results in a decrease in plasma retinol levels3,4 and concomitant diminished dark adaptation. Dermatological disorders were reported in a substantial number of subjects.5 
Glucuronidation of drugs and natural products is a common metabolic pathway that usually facilitates excretion.6 An important metabolite of 3 is 4-HPR-O-glucuronide (4-HPROG; compound 5) in which the phenolic hydroxyl group is linked to the sugar. Subsequent to its discovery, compound 5 was synthesized and evaluated for bioactivity, and was shown to have excellent chemopreventative activity in a rat mammary tumor model.7 However, it was not determined if the glucuronide 5, which was shown to be hydrolyzed to compound 3 via β-glucuronidase,8 was advantageous due to improved bioavailability of 3 or had activity in its own right as intact 5. To study this issue, an enzymatically stable glucuronide analog was synthesized by replacing the phenolic oxygen with a methylene group. The carbon-linked analog 4-HPR-C-glucuronide (4-HPRCG; compound 6) was shown to have excellent chemopreventative9 and chemotherapeutic10 properties. Furthermore, much like 4-HPR, compounds 5 and 6 show low affinity relative to RA for binding to the nuclear retinoic acid receptors (RAR), which mediate most of the actions of natural retinoids.9 Unlike compound 2,4-HPR causes apoptosis in numerous cancer cell lines.11 Thus the mode of action of these synthetic retinoids remains unclear.

While 4-HPR (compound 3) has been shown to be an effective chemopreventative and therapeutic agent, some of its effects may be attributed to in vivo hydrolysis of the amide bond, liberating RA. To investigate this possibility, an unhydrolyzable analog of 4-HPR, 4-hydroxybenzyl retinone (4-HBR; compound 4) was synthesized. Both compounds 3 and 4 were shown to be equiactive chemotherapeutics in the dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumor model.12,13 In vitamin A-deficient rats, compound 3, but not compound 4, is hydrolyzed to liberate retinoic acid. 13 Furthermore, 4-HPR (3) but not the C-linked analog (4) induces CYP26B1 mRNA expression in a RA-like manner in the lungs of vitamin A-deficient rats. Based on the positive chemotherapeutic and apoptotic-inducing activity of compound 4, it appears that hydrolysis of 4-HPR is not required for the therapeutic effect of this retinoid, but rather, the liberation of RA may contribute to its retinoid-based toxic side effects.
4-HPR has been shown to be 100 times less teratogenic than RA and this toxicity may also be caused by the liberation of RA. With the effective antitumor agent 4-HPRCG (compound 6), amide bond hydrolysis may still occur in vivo, thus liberating retinoic acid by similar mechanisms as for 4-HPR. Therefore, an unmet need exists for compounds that exhibit the desirable anti-neoplastic activities of HPRCG, but which are not metabolized in vivo to yield retinoic acid. The present invention is directed to such compounds.